This invention relates to novel hot melt adhesives and to methods of making them.
Hot melt adhesives are widely used in connection with paper products, e.g., for binding a stack of paper sheets together along one edge to form a book, sealing the flaps of conventional corrugated cardboard boxes, etc. Such adhesives, which are normally tack-free solids at room temperature, become fluid when heated, enabling them to be quickly applied and equally quickly solidified to form a firm adhesive bond.
Because of the growing emphasis on recycling waste or scrap fiberboard products, there is a commercial appetite for a hot melt adhesive having the additional characteristics of being soluble or dispellable in water. With such an adhesive, fiberboard products having hot melt adhesives in or on them could be thrown into a conventional fiberboard box repulping operation, where the adhesive would be dispersed or dissolved. Such an adhesive would thus avoid the formation of lumps which would clog screens in the repulping machinery or appear in the final fiberboard as blotches which may weaken the product or cause it to be unsightly. On the other hand, it is important that the adhesive be reasonably insensitive to atmospheric moisture and stable at high temperatures for long periods of time in order to endure commercially practical application techniques.
Various attempts have been made to provide a water-soluble hot melt resin having the aforementioned properties. For example, U.S. Pat. No. 3,597,264 discloses a remoistenable -- but not water-soluble -- hot melt which is a blend of partially hydrolyzed polyvinyl acetate and plasticizer. U.S. Pat. No. 3,753,944 discloses a water-soluble hot melt which is a blend of high and low molecular weight polyoxyethylene compounds and related materials, but this product is soft and not sufficiently strong for use in many of the applications where hot melts are employed.
U.S. Pat. Nos. 3,734,874 and 3,779,993 disclose water-dissipatable meltable polyester adhesives formed by reacting monomer components which include 100 moles of dicarboxylic acid and 100 moles of diol, at least 15 mole % of the diol being poly(ethylene glycol), some of the difunctional monomers containing a metal sulfonate group attached to an aromatic nucleus. While these adhesives may be effective in some applications, they suffer degradation upon prolonged exposure to the high temperatures of practical adhesive processing techniques and lack resistance to high humidities.
It has been found by others that water-dispellable hot melt resins can be made by reacting certain diols with alpha, beta-unsaturated dicarboxylic acid ans thereafter reacting the unsaturated polyesters with solubilizing agents. These resins are extremely useful for book binding, but the two-step manufacturing process requires great care to arrive at products having the viscosity required for fiberboard box sealing.
It is believed that prior to the present invention, there existed no easily prepared hot melt adhesive possessing the combined attributes of high strength at a wide range of humidities, resistance to high temperatures aging, and water solubility or dispellability.